There are many electronic applications where it is desirable to have a circuit which generates a current that varies as a function of temperature, and such circuits exist. However there are other applications where it is desirable for the current generating circuit to be adjustable, to be able to achieve a linear temperature dependence, and to be suitable for use in integrated circuitry.
For example, an instrumentation circuit for an automobile or other vehicle may rely on a transducer input. The transducer supplies a signal to an amplifier circuit in response to an input stimuli. This signal typically has a DC offset voltage, and may vary with temperature as the transducer may have a temperature dependent characteristic. At the output of the amplification circuit is the amplified transducer signal. What is desired is a signal proportional to the input stimuli to the transducer but independent of temperature. This may be accomplished by establishing a gain in the amplifying section that is temperature dependent in such a manner as to compensate for the temperature variation in sensitivity of the transducer. Typically, a summation stage is added for further offsetting the output following amplification such that the output is in a usable voltage range.
If the transducer in the above circuitry is of the piezoresistive type, it may exhibit sensitivities that decrease with increasing temperature in a hyperbolic fashion. In these cases it is desirable to have a corresponding amplifier gain that increases linearly with temperature such that the overall transfer function is temperature independent. However, problems arise when the signal from the transducer includes an offset term, as this term also is multiplied by the temperature dependent gain term. Although the offset of the transducer may typically be temperature independent, when multiplied by the temperature dependent gain of the amplifier, the offset becomes temperature dependent.
One known method of eliminating temperature dependence of the offset term appearing at the output is by employing a means of trimming out the offset in the transducer. In many cases, however, this is not practical because the exact value of the transducer offset may not be known until the amplification and compensation circuitry has been added. In those cases, another method of offset compensation may be desirable.